Screen grid tube



Dec. 17, 1940. M. HARNascH SCREEN GRID TUBE Filed Oct. A'22, 1938 INVENTOR M. /wsf/P/v/fcw.

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t B M 7M -l 9 Z Patented Dec. 17, 1940 UNITED "STATESv PATENT GFFICE i SCREEN dnrli'lfuisnrA Martin Harnisch, Berlin-Siemensstadt, Germany,

assignor to Fides Gesellschaft fr die Verwaltung und- VerwertungV von gewerblichen Schutzrechten mii-,beschrnkter Haftung, Berlin, Germany, a corporationof Germany-A Appneation october 22, 1938, serial No. 236,433 In Germany January 22, 1938' s. claims. (ci. 25m-271.5) v

The invention relates to. discharge devices and especially to the type of discharge device having multiple electrodes such as the screen grid tubes, pentodes .and so forth..

An object of the invention isto providev asubstantially linear characteristic between the. an.- ode current and the grid potential Vin the above mentioned tubes, andespecially such tubes: used as amplifier tubes.

Other objects and4 advantagesof the invention .will be apparent fromthe following description and drawing in which:

Fig. 1 is a graph illustrating the` characteristic between the anode current and the grid potential in tubes of the prior art.

Fig. 2 is a cross sectionalview of a. dischargek device disclosing a preferred embodimentof the;

invention. Y Fig. 3 is a graph disclosing the.4 linear characteristic between the anode current-and grid potential of the devicein Fig. 2.

Fig. 4 is a cross sectional view througha duplex set of electrodes embodying the. invention.

embodying the invention and having a multiple set of electrodes.

It is highly desirable to have alinear characteristic between the anode current. and thev grid potential throughout the range of. current values for the tube. Unfortunately, however, as established by Langmuir, the current .in .the anode circuit is proportional tothe root ofthe third power of the control grid potential. Where additional 1 g grids or auxiliary electrodesare utilized in addition to the control electrode, the linearity of the tube characteristic is further disturbed by the presence of these additional grids. The distribution of the cathode current between the screen grid and anode does not take place in the same ratio for operating conditions, but is dependent upon the magnitude of the screen grid potential and the anode potential or the effective potential in the space between the two. By proper selection of the external resistance, the upper portion of the tube characteristic, in regard to anode current and grid potential, can be made satisfactorily linear.

In Fig. 1 the anode current is plotted as ordinate and the grid potential is plotted as abscissa. The curve I illustrates the characteristic of anode current to grid potential for triodes and screen grid tubes without external resistance, as established by the Langmuir Discharge Law. By the use of external resistance, the upper portion Fig. 5 is. a cross sectional View through a tube of the curve can be made substantially linear as illustrated in the heavy black line extending above the operating point 2. The portion of the curve line, below the operating point2 has, however, a relatively large curvature 3 and it is one of the 5 objects of the present invention to change this relatively large curvature into a substantially linear `.relationship so. that the line will be substantiallyV linear throughout the various anode currentvalues of the tube. n

In Fig. 2is illustrated a preferred 4embodiment of the inventionfor obtaining a substantially linearcharacteristic of the tube. In the drawing', thenumeral 4 Yis thecathode which, as illustrated, is formedV as substantially an arc segment having 10 a concave electron emittingsurface. The control gridI 5 and the screen grid 6, if used, are also preferably curved segments concentric with the concavesurface ofthe cathode 4. Atsubstantialf 1y thenormal focal pointof the concave cathode A and'itsassociated control and screen grids, 5 and 6, is located an auxiliary electrode 1. This auxiliary. electrode] is, preferably in' the form of a perforated plate, or mesh, lyingsymmetrically 2, withrespect to the curved cathode and thecontrol and screenl grids., The` plane of the yauxiliary electrode ,l is .within the plane bisecting the curved segment 4 of theV cathode. The anode 9 isalso preferably in the form of a plate lying within the same plane as vthat of the auxiliary electrode l, which plane, as previously mentioned, bisects. the curved segment of the cathode. This positionof the anodev 9 makes a lower capacity between the anode-and the other elements. Intermediate. the auxiliary electrode l andthe anode-9 maybe located a second, screen grid 8. If desired, the grid 8 may be utilized as a braking or suppressor grid. The auxiliary electrode 'l has preferably a xed positive potential.

The device of Fig. `2 operates in the following manner: The curvature of the concentric electrodes is selected that for smaller currents, the intersection point of the electron streams falls on the auxiliary electrode 1. The auxiliary electrode 1 accordingly has a comparatively large current because it is impinged upon by many electrons that are absorbed thereby, instead of being passed on to the anode. If the tube current is made larger, then the electron stream is broadened out and the concentration point is displaced towards the anode. The current which the auxiliary electrode absorbs becomes continually smaller with increasing tube current. Finally, for large currents, a substantially smaller portion of the current is taken up by the auxiliary electrode. It is possible by the selection of the position of the auxiliary electrode, its size and its construction, to inuence the relationships in such manner that the current drawn through the auxiliary electrode is in the desired ratio to the tube current. Therefore, the lower portion of the characteristic of the tube between anode current and grid potential may be made substantially rectilinear. The radial position of the plane anode in respect to the cylindrical segment of the cathode 4 provides a. broad region for the focusing and attracting of electrons thereto. Heat of the anode is also more effectively radiated.

These relationships are illustrated in Fig. 3.

Instead of the curved portion I0, the tube, con

structed according to the invention disclosed in Fig. 2, will have a straight line characteristic II in its lower portion. This straight line characteristic is provided by the auxiliary electrode taking a current which is the magnitude of the difference of the current values of the characteristics between the curved line IIJ and the straight line II illustrated by the iine black lines extending between the two curves for various values of grid potential.

The use of the second screen grid 8 between the anode 9 and the auxiliary electrode 'I is of advantage, because it shuts out the influence of the a-node potential and makes the position of the intersecting point on the characteristic curve independent of the magnitude of the instantaneous anode-potential. Because of this fact, the invention, from a practical standpoint, is especially adapted for screen grid tubes. It will be noted, also, that the desired rectilinearity of the characteristic may be obtained without any decrease in the steepness of the curve.

Either a directly heated or an indirectly heated cathode may be used. However, it is preferred to use an indirectly heated cathode since the desired concave curvature can be easily formed and then coated with the usual electron emitting coating.

Fig. 4 discloses a further modification of the invention, utilizing a. duplex set of electrodes. In this disclosure, the second screen grid also serves as a diaphragm for the purpose of better focusing the electron rays. No lost currents are absorbed by this second screen grid. In this gure, the cathode I2 is illustrated as having two concave electron emitting surfaces, emitting in opposite directions. Adjacent to each of these surfaces is the concentrical curved control grid I3 and the normal screen grid I4. The auxiliary electrode I has its position similar to that illustrated in Fig- 2. The second screen grid I6, however, is constructed as a diaphragm for concentrating the electron stream to impinge upon the anode I8. The braking or suppressor grid II may be used if desired. The dotted lines in Fig. 4 illustrate diagrammatically the more crowded electron paths coming from certain cathode areas with increased current over that in Fig. 2, and the more distant focus of these electron paths beyond the auxiliary electrode.

Fig. 5 has a schematic showing of a plurality of electron systems joined together in a suitable manner to form a unitary system for high powered tubes. The cathode is formed of four curved surfaces, I9, 2U, 2l, and 22, which are joined together as illustrated with electron emitting paths concentrated in four different directions. The corresponding control grids, as well as auxiliary electrode and anodes, are illustrated corresponding to those of Fig. 2. The cathode may, of course, be either directly or indirectly heated. The invention illustrated in Fig. 5 has the advantage, however, that a centrally arranged heating body may be used for all four of theelectron emitting surfaces, if an indirectly heated cathode is utilized. The corresponding electrodes in the systems in Fig. 4 and 5 may be connected together within the tube if desired.

It is apparent that many modifications may be made in the form, number, and arrangement of the elements disclosed Without departing from the spirit of the invention.

I claim:

1. A discharge device comprising an envelope containing an anode, and cathode, said cathode having a concave electron emitting surface, and an auxiliary electrode having a surface for the impingement of electrons thereon substantially at the focal point of said concave surface, and a screen grid intermediate said auxiliary electrode and said anode, said screen grid having an opening therethrough for directing electrons to said anode.

2. A discharge device comprising a concave cathode, a concave control electrode and a plane anode, and an auxiliary electrode intermediate said anode and the group of said concave cathode and control electrode, said auxiliary electrode being in a plane aligned with said plane anode.

3. A discharge device comprising an envelope containing a cathode having a surface whose section is an arc, a plane anode and a plane auxiliary electrode, said anode and auxiliary electrode being in a plane substantially bisecting said arc and also being spaced from each other in said plane.

. MARTIN HARNISCH. 

